1. Field of the Invention
This invention generally relates to apparatus and methods for collection of data from a number of remote devices or sensors. In particular, embodiments of the invention employ a communications protocol designed to resolve contention and allow for collection of data from a variable number of devices using a mobile data collection platform employing single frequency RF signals.
2. Description of Related Art
The need for a system to provide for automated collection of data from remote sensors has been recognized for many years. Such systems can be used in a variety of contexts, such as for monitoring remotely located sensors from a central station or mobile transmitting and receiving unit, e.g. for monitoring utility commodities such-as gas, water, electricity and the like. one such system is described in U.S. Pat. No. 4,614,945 to Brunius, et al. (incorporated herein by reference).
Systems currently in use for these types of applications include telephone, hardwired, and RF based systems. Telephone based systems employ either a dial-in mechanism, whereby a remote sensor dials up a computer system and transfers data, or a dial-out system whereby a central computer dials sensors and initiates data transfer. In either case, dial-in or dial-out, these systems require data collection devices to be connected to a telephone line. This complicates installation due to dependence upon a third party, i.e., the telephone utility company. Current technology provides for detecting whether or not the phone line is in use prior to initiating data transfer. Once the data transfer is in progress, the user is precluded from accessing the line until data transfer has been completed. Interruptions such as a pick-up of a phone during data transfer or activation of the call waiting feature can introduce data errors and necessitate the retransmission of data.
In the case of hardwired systems such as those used for electric meters, existing power lines are used as the communications media for the transfer of data. Such systems typically require the use of communications concentrators to store and forward data and require replacement of existing meter devices. Expensive to install, these systems are also dependent upon the integrity of existing lines. Loss of one or more communications concentrators or a damaged power line can result in total or partial system failure.
RF based systems provide advantages because they are not dependent upon existing telephone or power lines. RF based systems can employ distributed concentrators for collecting, storing, and forwarding data to a centralized facility; or employ an RF equipped mobile data collection platform which collects data from remote sensors along the route of travel. Typical mobile collection platforms send out interrogation signals to RF equipped remote sensors which in turn use RF signals to send back data packets containing their respective ID and data. Fundamental to their successful operation is a method of dealing with the contention introduced by a varying number of RF equipped sensors within range of the platform. Existing RF communications techniques typically employ polling schemes whereby the mobile data collection platform transmits a data packet containing the ID of the sensor which is to be interrogated. Given that the RF transceivers have a limited range, this technique requires that mobile platform follow a specific route wherein identities and sequencing of the remote sensors is known in advance. This approach makes such a system dependent upon vehicle speed, data transmission rate and route (i.e., a specific sequence of devices to be read must be known). Deviations in route or vehicle speed can introduce failures by disrupting the sequencing or timing associated with communications protocols.
Another technique employing a mobile data collection platform is described in U.S. Pat. No. 4,614,945 to Brunius, et al. This patent describes a method for data collection whereby each sensor within broadcast range of the mobile unit sends its identification code and accumulated data a plurality of times by serially spaced transmission bursts. The time interval between successive transmission bursts of different sensors/transponders differs. All sensors respond to a common "wake-up" signal and immediately broadcast their transmission bursts of their identifier and accumulated data back to the mobile unit. This technique does not uniquely poll individual transponders. In order to minimize collision interference between signals of simultaneously transmitting devices, ". . . the transponders change the active time and frequency parameters of their respective RF transmissions." In addition, the frequency for each transponder can also vary as a function of the unique identifier for a given transmission interval. Reference is also made to use of a randomizing function to determine time intervals and transmission frequencies for transponders. This technique is intended to avoid collisions between transmission bursts from the plural transmitting of other transponders by actively varying both the time interval and transmission frequency for each active transponder.
The communications scheme described in the Brunius, et al. patent requires that extreme care be used when installing instrument sets in the field to ensure that two or more transponders operating on the same frequency are not installed within the same broadcast range. It also requires a complex mobile unit with receiver units to match instrument sets that are expected to respond within a given operating range. This immediately suggests problems with overlap of frequencies since the data collection platform is mobile. Such overlap of frequencies can adversely affect the collision avoidance scheme. The complex nature of the scheme and its dependency upon the spatial placement of instrument sets, suggests a complex and expensive installation procedure, in addition to a high cost for system operation and maintenance.
Several communications techniques involving RF communications for the purpose of collecting data from remote sensors using a mobile data collection platform have been developed. However, such techniques typically do not mitigate problems associated with the dynamics of a mobile data collection platform, namely variances in vehicle speed, route, and data transmission rate. Embodiments of the communications protocol described below were specifically developed to utilize the physical dynamics of a mobile RF transceiver communicating with a plurality of stationary RF transceivers operating on the same frequency.